The present invention relates to superconducting motors and more particularly to a reduced size rotor construction including torque tubes positioned inside a rotor support.
Generally, referring to FIG. 1, an AC synchronous superconducting motor 200 includes a rotor support 202 mounted on a rotor shaft 204a and 204b. Rotor windings 206 are arranged around the support forming a rotor assembly. The assembly is mounted inside a stator cavity 208. The stator includes a plurality of stator windings 210 arranged to form cavity 208. A DC current is provided to rotor windings 206 which generates a rotor field inside cavity 208. An AC current is provided to stator windings 210 which generates a magnetic field therearound located at least partially within cavity 208. By time varying the AC current, the stator field is caused to rotate about cavity 208. The rotor and stator fields interact and, as the stator field rotates about cavity 208, the rotor follows.
Three important motor criteria for any type of motor are size, power output and efficiency. High power and reduced size are desirable without compromising efficiency. These three criteria do not go hand in hand. For example, the simplest way to increase motor output is to increase stator and rotor currents. Increased currents result in stronger stator and rotor fields and therefore stronger attraction between the fields. Unfortunately, as currents are increased, so to is the heat generated by the currents as the currents pass through the stator and rotor windings. Energy spent to generate this heat constitutes the majority of input energy losses. Thus, higher heat means lower motor efficiency. At some point, stator and rotor current levels reach a value where the efficiency drops below specified level. In these cases, generally, to increase output further, motor size must be increased.
Recent advances in superconductivity have enabled engineers to design synchronous motors which can theoretically generate relatively high power output for their size when compared to conventional motors. To this end, some motors have been developed which include superconducting rotor coils capable of carrying massive amounts of current through relatively few windings, thus reducing rotor girth and length and thereby reducing overall motor size.
In order to facilitate superconduction, conductors have to be extremely cold (i.e. approximately 5.degree. K. for low temperature superconductors). To this end, referring again to FIG. 1, superconducting rotor supports 202 are located inside a vacuum jacket 209 and connected to first and second shaft ends 204a and 204b, respectively, via first and second torque tubes 212a and 212b, respectively. At least one of the shaft ends 204b forms a passageway 214 from an inlet to an outlet which opens into one of the torque tubes 212a or 212b. The inlet is connected to a refrigeration system 216 which provides a cooling agent (e.g. liquid or gaseous helium) to the support 202 via a supply pipe 207 as well known in the art.
Torque tube configuration is generally governed by tube functions. Tubes are usually formed of stainless steel and have a wall thickness and radius which are sufficient to withstand shaft torque. To minimize heat transfer from shaft ends 204a and 204b to support 202 via the tubes 212a and 212b, tube cross-sectional area is typically kept to a minimum (i.e. tube cross-sectional area acts as a "heat bottleneck" limiting heat transfer). At the same time, tube length is typically relatively long as longer tubes limit heat transfer therethrough.
Unfortunately, typical tube construction and the typical shaft end-tube-support configuration often minimizes the advantages associated with superconducting motors. Because tubes 212a and 212b are provided between shaft ends 204a and 204b and support 202, the tubes directly increase overall motor size. For example, where each tube 212a and 212b is eight inches long, overall motor length L.sub.1 (see FIG. 1) must be increased by sixteen inches. Thus, tubes 212a and 212b directly minimize the size advantage associated with a super conducting motor.
One solution to reduce tube heating would be to form tubes 212a and 212b, at least in part, out of a heat insulating material such as glass-epoxy composite. To this end, each tube may be formed of a composite tubular member having first and second ends to be connected to a shaft and support, respectively. It has been found that to withstand typical motor torque, composite fibers should be configured about a tube rotation axis at an angle (e.g. 45.degree.) with respect to the axis such that the fibers extend at least partially axially. These tubes have to be connected to the metal vacuum jacket via a composite/metal joint. The length of the composite/metal joint increases overall motor size.
Such joints may include bolts, rivets, or the like tightened onto the composite in the radial direction (i.e. through the composite member wall essentially perpendicular to the rotation axis and fiber lengths). To this end, each tube may also include first and second metal plate ends which bolt axially to the shaft and support, overlap the composite member axially over a connection distance and radially either internally or externally of the member wall and are bolted radially to the member wall over the connection distance.
While heat transfer can be minimized in this manner, motor size is not. This is because the connection distance required to provide a sufficiently strong joint between the tube and plate ends is relatively long. For example, each connection distance may be on the order of 4 inches. Because there are four joints (i.e. one at each end of each of the two tubes), the joints will often increase motor length by as much as 16 or more inches.
Therefore, it would be advantageous to have a torque tube construction which minimally adds to overall motor size yet isolates the support from the shaft ends.